Screws wherein a metal wire such as a steel wire is helically wound around a flexible rod are used for transporting containers. A container transport apparatus using screws having such a structure is composed of two screws arranged in parallel at a specified distance from each other and are adapted to transport containers by rotating the screws, with tile neck of each container being held between the screws (usually with a flange at the neck of the container in engagement with shafts).
The apparatus transports containers by a principle entirely different from that of belt conveyors or roller conveyors which are usually used for container transport, i.e., by the frictional contact of the screws with the neck of the container, and permits more accurate container positioning, more facilitated automation of production lines, its simpler and more compact construction and lower equipment costs, as compared with conventional conveyors.
In the screw above, the wire helically wound is made of steel, and the neck of each container including its flange to be brought into contact with the wire is subject to abrasion or damage by rotation. Especially, great care should be given to the containers to be filled with a carbonated beverage to avoid flaws in the container.
For solving this problem, it has already been considered to use, in the place of the steel wire, a cord prepared by coating a polymer on a core of a bundle of twistings such as nylon. Since the surface layer of the cord is made from resin, the cord is advantageous in that the damage to the containers can be made less than do steel wires.
Cords having a core are generally produced by feeding a core in a direction under a given tension, extruding a polymer in a molten state onto the core from a die disposed coaxially therewith simultaneously with the advance of the cord to coat the core with the polymer, and cooling same. As the molten polymer solidifies, however, a buckling of the core occurs due to the difference in shrinkage between the polymer and the core. Upon application of a tension, the cord with buckling can be easily stretched for the same length as the core shrank.
According to the above-mentioned transport apparatus, containers are, with their own rotation, transferred by the sliding friction caused by a high speed rotation of the screw, which in turn means that the force to the direction which causes expansion of the winding pitch, namely, expansion of helical intervals of the cord acts repeatedly on the cord when containers advance on the cord. Accordingly, the cord is elongated on the rod to become loose in the course of operation due to the property of the cord mentioned above, thus resulting in an undesirable change in the winding pitch of the cord. The disturbance in the pitch makes container transport speed unsteady, and in an extreme case, the transport becomes impossible.
In view of the foregoing problems, it has been already proposed to use a polymer cord wherein the amount of the buckling of a core, i.e. a fiber bundle, is not more than 3.0% (WO 91/06493).
Out of the necessity to cope with the increasing force applied on a cord during transport of containers, which is caused by the growing container volume, and to meet the desire to develop a cord adaptable to a drastic change in the temperature of the environment in which the cord is used, requirements imposed on the cord have been becoming severer.
Accordingly, an object of the present invention is to provide a flexible screw for conveyors, which is free of disturbance in pitch caused by the growing container volume, and which can cope with changes in the temperature of the environment wherein the screw is used.